The performance of capacitor voltage transformer (CVT or CCVT) is inferior to that of electromagnetic voltage transformer. Its performance is affected by the supply frequency, switching transients, magnitude of connected burden, etc.
The capacitor voltage transformer is more economical than an electromagnetic voltage transformer when the nominal system voltage increases above 66 kV.
The carrier current equipment can be connected via the capacitor of the Capacitor Voltage Transformers. Thereby there is no need of separate coupling capacitors.
Capacitor Voltage Transformers also serve as coupling capacitors for coupling high frequency power line carrier signals to the transmission line.
CVTs in combination with wave traps are used for filtering high frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network.
Working of CVT
However, the burden current becomes relatively larger and ratio error and also phase error is introduced. Compensation is carried out by ‘tuning’.
The reactor connected in series with the burden is adjusted to such a value that at supply frequency it resonates with the sum of two capacitors. This eliminates the error.
The construction of capacitor type VT depends on the form of capacitor voltage divider. Generally, h.v. capacitors are enclosed in a porcelain housing. A large metal sheet box at the base encloses the tuning coil intermediate transformer.
Construction of CVT (CCVT)
Capacitor Voltage Transformers consist of two primary assemblies,
- the high voltage capacitor sections and
- the base box, housing the electromagnetic components.
The General arrangement of a capacitor voltage transformer is given below,
|General Arrangement of Capacitor Voltage Transformer|
|Schematic Diagram of CVT|
Main applications of CVTs in HV Networks
The main applications of Capacitor Voltage Transformers in High Voltage Networks ( above 36 kV ) are given below
- Voltage Measuring: They accurately transform transmission voltages down to useable levels for revenue metering, protection and control purposes.
- Insulation: They guarantee the insulation between HV network and LV circuits ensuring safety condition to control room operators.
- HF Transmissions: They can be used for Power Line Carrier (PLC) coupling.
- Transient Recovery Voltage: When installed in close proximity to HV/EHV Circuit Breakers, CVT’s own High Capacitance enhance C/B short line fault / TRV performance.